2.5 billion years ago, the Sun was basically invisible from the Earth's surface. Microbes in the oceans pumped methane into the atmosphere, creating a giant cloud of smog that covered the entire planet. Yes, the whole world turned into LA.

If you were to visit Earth at most points in its history, it wouldn't look that different. The continents might be rearranged, but you would still recognize our planet as that familiar pale blue dot. But until about 2.45 billion years ago, Earth was locked in a constant cycle of flipping between clear skies and this hydrocarbon smog. Earth's atmosphere would have looked much like that of Saturn's moon Titan, which is also covered in a methane fog.

This theory of Earth's atmospheric past has been around for a while, but it's only now that an international team of researchers have been able to find definitive proof. By analyzing sediments found in South Africa dating back 2.5 to 2.65 billion years, the team were able to reconstruct the unique atmospheric cycle that then dominated the planet. Writing in Nature Geoscience, they explain the strange push-pull that governed the skies of the early Earth:

We find evidence for oxygen production in microbial mats and localized oxygenation of surface waters. Carbon and sulphur isotopes indicate that this oxygen production occurred under a reduced atmosphere that was periodically rich in methane, consistent with the prediction of a hydrocarbon haze. Our simulations predict transitions between two stable atmospheric states, one with organic haze and the other haze-free. The transitions are presumably governed by variations in the amount of biological methane production during the Archaean eon.

This cycle likely came to an end with the rise of cyanobacteria, which produced enough oxygen to overwhelm the methane-producing microbes and end the era of hydrocarbon hazes. Still, if you're looking for a fresh spin on the post-apocalyptic story, how about finding a way to restart this cycle of thick, impenetrable methane smog? Of course, as an LA resident, that just sounds like Tuesday, but I bet others would find it terrifying.